1. Field of the Invention
This invention relates to circuits for detecting frequencies other than a selected fundamental frequency in an ac electrical signal. More particularly, it is directed to such detectors utilizing discrete time analog filters which can be switched at high frequency to operate as a HIGHPASS filter to detect frequencies above the fundamental frequency or switched at the fundamental frequency to alias the fundamental and its harmonics to detect the presence of other frequencies. The circuits have particular application in detecting extraneous frequencies in variable speed constant frequency (VSCF) generators.
2. Background Information
In certain applications it is desirable to detect frequencies in an ac signal other than the fundamental frequency. One such application is the solid state variable speed constant frequency (VSCF) generators used, for instance, in airborne power systems. Such VSCF generators utilize solid sate inverters to synthesize an ac signal, usually from square wave pulses. The ac signals generated by such devices typically have a sizable harmonic content, especially the odd harmonics. It is common to filter the synthesized ac signals to reduce this harmonic content. Until recently, specifications have called for means for detecting frequencies above fundamental in the filtered ac power signal. The presence of harmonic noise in the filtered ac signal could indicate a failure within the filter. Non-harmonic noise is possibly an indication of a failure in the generator or of drift in the frequency of the generator. Currently, high pass filters composed of discrete components are used to detect frequencies above the fundamental. A number of components are required to achieve a suitable corner frequency in the response characteristic above the fundamental frequency.
Specifications have recently been modified to require detection of frequencies in the ac output of VSCF generators below as well as above the fundamental frequency. This could be achieved by providing a low pass filter which passes frequencies below the fundamental frequency in addition to the high pass filter, although this would add to the complexity, size, weight and cost of the system, all of which are factors to be minimized in an airborne system.
There is need therefore for an improved apparatus for detecting frequencies both above and below the fundamental frequency in an ac signal.
There is also a need for such an apparatus which minimizes size, weight, complexity and cost.
There is a further need for such an apparatus which can be implemented by an integrated circuit.